1. Technical Field
This invention relates to sodar methods and apparatus for sounding in the lower atmosphere and is particularly applicable to ‘monostatic’ systems where backscattered echoes are detected by receiver(s) located near the transmitter. However, in some embodiments, this invention is also applicable to ‘bistatic’ systems where forward-scattered echoes are detected by receiver(s) located remotely from the transmitter (the distances being relative to the sounding range).
It is appreciated that a more narrow usage of the terms monostatic and bistatic can be found in the art in which ‘monostatic’ indicates systems in which the same aerial is used for transmission and reception (as is common in radar) and ‘bistatic’ being used for systems where the transmit and receive aerials are separate. This narrow usage is not helpful in sodar where ‘listen-while-sending’ techniques may be employed and is not used herein.
In this specification, the term ‘chirp’ is used as convenient shorthand for an interrogation or chirp used in sodar sounding that is acoustic and is encoded or modulated in a manner adapted to facilitate the extraction of the returned echo components of received acoustic signals.
2. Discussion of Prior Art
In our prior U.S. Pat. No. 6,755,080 and our prior international patent applications PCT/AU2002101129, PCT/AU2004/00175 and PCT/AU2004/00242 we addressed the central problem of low signal-to-noise (s/n) ratios in sodar by the use of (i) long chirps, (ii) ‘listen-while-sending’ techniques in which transmission overlaps reception and (iii) matched filtering to extract echo data from the received signals by making use of the encoding of the chirp. The discussion of the prior art in this patent and in these applications is incorporated herein.
In the context of the present invention, a ‘long’ interrogation—or transmit—pulse is taken to be one that has not terminated before the first echoes of interest are received; hence the need for the listen-while-sending technique. In absolute terms, chirps of between 100 ms and tens of seconds with bandwidths of 3-10 kHz are preferred. The great advantages of our listen-while-sending technique are (i) the very large processing gain made possible by the use of the long chirp, (ii) the elimination of the need for the high peak powers associated with short pulses and (iii) the removal of the range and power limitations of send-then-listen sodar. We found that listen-while-sending was entirely practical in bistatic sodar because the combination of low peak transmit power, high dynamic-range receiver microphones, good acoustic shielding of receivers from the direct signal and the high system processing gain allowed echoes to be readily detected ‘behind’ the direct signal. [The direct signal is that which is transmitted directly between transmitter and receiver without reflection from the atmosphere.]
Our prior patent applications disclosed other advantageous sodar techniques such as the selective combination of the outputs of multiple matched receivers to substantially eliminate undesired signal components and unique methods of processing extracted echo data to generate information of value to meteorologists and air-traffic controllers. Though our prior patent applications were not limited to the use of Fourier—or frequency domain—matched filter techniques, the practical examples provided illustrated such techniques because (i) the implementation of time-domain matched filter methods were computationally demanding and impractical for the prompt generation visual displays when using long chirps, and (ii), they were found to be inferior to the computationally efficient Fourier methods where Doppler echo components were of importance.
We have now been surprised to find that appropriate time-domain matched-filter processing of received signals can be implemented without excessive computing power in both bistatic and monostatic systems. We have also found that two other factors facilitate this, particularly in the more challenging monostatic systems: first, attention to the arrangement and alignment of transmitter-receiver pairs and, second, the use of narrow bandwidth chirps. These appear to significantly improve recovery of Doppler components from received signals when using time-domain matched filtering with either FM (frequency modulated) or DM (digitally modulated) chirps. Indeed, these factors also improve sodar systems using encoded chirps and Fourier domain matched filtering.